[PATCH] libstdc++: Implement P3060R3: Add std::views::indices(n)

[Jonathan Wakely]

On Mon, 20 Oct 2025 at 15:04, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> On Wed, 15 Oct 2025 at 16:23, Yuao Ma <addr2line@gmail.com> wrote:
> >
> > Hi Jonathan,
> >
> > Thanks for the information about the standard wording. Regarding this
> > patch, I'm wondering does it need any further refinement?
>
> No, I don't think you need to make any changes.

I see this was pushed to trunk - please wait for explicit approval.

My last mail probably wasn't very clear, but I hadn't finished a final
review. Not a big deal, since the patch is good.

It turns out another change is needed though, because the test failure
you got with __max_size_type should be SFINAE-friendly, because
indices(1) should be expression equivalent to iota(0, 1).

I'm testing this fix:

Author:     Jonathan Wakely <jwakely@redhat.com>
AuthorDate: Tue Oct 21 21:21:45 2025
Commit:     Jonathan Wakely <redi@gcc.gnu.org>
CommitDate: Tue Oct 21 21:21:45 2025

   libstdc++: Add missing constraints to views::indices

   Calling views::indices(n) should be expression equivalent to
   views::iota(decltype(n)(0), n), which means it should have the same
   constraints as views::iota and be SFINAE-friendly.

   libstdc++-v3/ChangeLog:

           * include/std/ranges (indices::operator()): Constrain using
           __can_iota_view concept.
           * testsuite/std/ranges/indices/1.cc: Check SFINAE-friendliness
           required by expression equivalence.

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 25d2e28e72ff..158692d92a70 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -791,6 +791,7 @@ namespace views
  struct _Indices
  {
    template<ranges::__detail::__is_integer_like _Tp>
+      requires __detail::__can_iota_view<_Tp>
      [[nodiscard]] constexpr auto
      operator() (_Tp __e) const noexcept
      { return iota(_Tp{}, __e); }
diff --git a/libstdc++-v3/testsuite/std/ranges/indices/1.cc
b/libstdc++-v3/testsuite/std/ranges/indices/1.cc
index 805b29e358f7..0b5ed3a37afc 100644
--- a/libstdc++-v3/testsuite/std/ranges/indices/1.cc
+++ b/libstdc++-v3/testsuite/std/ranges/indices/1.cc
@@ -29,3 +29,17 @@ int main() {
  VERIFY(test<size_t>(44));
  static_assert(test<size_t>(44));
}
+
+template<typename T>
+constexpr bool test_wider(T n)
+{
+  // If indices(n) works, try again with ranges::distance(indices(n)),
+  // which will be a wider type, until we get to an unsupported type.
+  // This verifies that indices(n) is SFINAE-friendly, because otherwise we
+  // would get a hard error outside the immediate context checked by requires.
+  if constexpr (requires { std::views::indices(n); })
+    test_wider(std::ranges::distance(std::views::indices(n)));
+  return true;
+}
+
+static_assert(test_wider(0));